Deep ultra-violet (DUV) lithography is widely used in the fabrication of advanced VLSI (Very Large Scale Integration) semiconductor devices. Chemically amplified DUV photoresists improve the performance of the lithography systems and improve device feature resolution. Low dielectric constant (low-k) dielectrics are favored in today's semiconductor manufacturing industry because of the performance improvements they provide by way of reducing parasitic capacitance, reducing propagation delay and therefore increasing device speed. The use of copper interconnect features is also favored to reduce line resistance of the interconnect lines. Typical copper interconnect schemes incorporate damascene manufacturing techniques to define the interconnect paths. A dual damascene approach is favored because it provides lower cost processing, improved level-to-level alignment tolerance and thus allows for tighter design rules and improved performance.
A shortcoming associated with the use of low-k dielectrics in conjunction with copper interconnect lines and chemically amplified photoresists used in DUV lithography, is that base groups which become nested in porous low-k dielectric materials, can interact with the acid catalysts included in chemically amplified photoresists to render the exposed photoresist insoluble in developer. This insoluble photoresist distorts the pattern being formed and is difficult to remove. The distorted pattern may result in electrical opens because via and contact openings cannot be formed. Base groups such as amines and other N—H base groups, are typically produced in association with conventional hardmask films, etch-stop layers and barrier films used in the film stack that also includes low-k dielectric films, and which is advantageously used in dual damascene processing. Etch-stop films and barrier films are commonly nitrogen-containing films, and amine or other N—H base groups may be produced during the formation of such films.
The use of copper as an interconnect material requires the use of a barrier layer which typically includes nitrogen and is free of oxygen. The presence of oxygen in an adjoining film or during the formation of an adjacent film, undesirably causes the formation of copper oxides by reaction with copper. Copper oxides undesirably degrade adhesion which could lead to mechanical failure. Moreover, after copper interconnect lines are formed using damascene technology, for example, organic corrosion inhibitors are typically formed over the copper surface. The organic corrosion inhibitors prevent the formation of copper oxides and prevent corrosion from occurring while the substrate including the exposed copper film, is transferred from a polishing tool, for example, to a film deposition tool used to form films over the copper surface. A plasma chemistry including ammonia, NH3, is typically used to clean or treat the copper surface, remove any copper oxides which may form, and to remove the organic corrosion inhibitor. This ammonia-containing chemistry also produces amines or other N—H base groups which can diffuse into the porous low-k dielectric material and then into the photoresists.
It is therefore desirable to enjoy the benefits provided by copper interconnect lines, low-k dielectric films and chemically amplified photoresists in DUV lithography systems, without degrading the chemically amplified photoresist by interaction with base groups from the porous low-k dielectric film.